Chromium carbide type ceramic materials have been known to have many excellent characteristics, including excellent strength, hardness, anti-oxidation, anti-scaling, anti-erosion and anti-corrosion properties, they also exhibit excellent permanent non-magnetizability and surface lustrousness. As a result, chromium carbide type ceramic materials have been widely used in a variety of industrial applications, such as shaft bearings, shaft seals, high-temperature furnaces, nozzles, metal machining molds, measurement tools, etc. Typically, there are four species of the chromium carbide, namely Cr.sub.3 C.sub.2, Cr.sub.7 C.sub.3, Cr.sub.23 C.sub.6, and Cr.sub.3 C, of which Cr.sub.3 C.sub.2 is the most commonly employed. However, it has also been reported that chromium carbide ceramic material does not provide satisfactory property in fracture toughness, and, as a consequence, the reliability of the chromium carbide material is less than desired. This weakness severely limits the applicability of the chromium carbide ceramic material in harsh environment.
U.S. Pat. Nos. 4,927,791, 4,945,073, 4,963,183, and 5,141,571, and Japanese Laid-open Patent Application Nos. 53-89807, 59-107972, 60-145954, and 62-211340 disclose a variety of chromium carbide based composite materials. These composite materials contain at least two of the following starting materials: (A) 0.2.about.10 wt % of at least one nitride, such as CrN, TiN, TaN, NbN, ZrN, AlN, VN, Si.sub.3 N.sub.4 and BN; (B) 5.about.45 wt % of various species of chromium carbide other than Cr.sub.3 C.sub.2, i.e., Cr.sub.7 C.sub.3, Cr.sub.23 C.sub.6, and Cr.sub.3 C; (C) 0.5.about.50 wt % of aluminum nitride, AlN; (D) 45-90 wt % of tungsten carbide; (E) 10.about.40 wt % of more than one kind of carbide or nitride of a transitional metal, such as Ti, V, Cr, Zr, Nb, Mo, Ta, Hf, etc.; (F) 1.about.30 wt % of a metallic sintering aid (such as Ni, Cr, Co, Ti, etc.); and (G) stoichiometric balance of chromium carbide Cr.sub.3 C.sub.2. Although these composites exhibit improved mechanical properties than the single-component chromium carbide ceramic material, shortcomings still exist.
Japanese Laid-open Patent Application Nos. 59-107972 discloses a sintered chromium carbide body containing 0.2 to 10% by weight of AlN. Because the amount of AlN in the chromium carbide composite disclosed in the '792 patent is less than 10%, it only provides limited improvement in the flexural strength of the final produce, to between about 310 and 540 MPa.
U.S. Pat. No. 4,927,791 discloses a sintered chromium carbide body containing 99.5% to 50% by weight of chromium carbide and 0.5 to 50% by weight of AlN. The chromium carbide crystals in these composites have an acicular (needle-shaped) structure. Although the chromium carbide composites disclosed in the '791 patent provide significantly improved fracture toughness (6.2 MPa.cndot.m.sup.0.5 vs 5.5 MPa.cndot.m.sup.0.5 for the conventional relatively equal-axis chromium carbide crystals), its flexural strength, which is about 400 MPa, is still considered less than desired.
Japanese Laid-open Patent Application Nos. 60-145954 discloses a chromium carbide composite containing a mixture of various species of chromium carbide including Cr.sub.3 C.sub.2, Cr.sub.7 C.sub.3, Cr.sub.23 C.sub.6, and Cr.sub.3 C, with Cr.sub.3 C.sub.2 being the predominant component. In the composite disclosed in the '954 publication, the "other" species, i.e., Cr.sub.7 C.sub.3, Cr.sub.23 C.sub.6, and Cr.sub.3 C, are used primarily as sintering aids, they do not contribute to the improvement of the mechanical strength of the final composite material. Therefore, no observable improvement in the flexural strength or fracture toughness is manifested in the composite.
Composites containing tungsten carbide and chromium carbide have been disclosed in the art. However, in these composites, tungsten carbide is the principle component and at least one metallic sintering aid is required in the preparation of the composite. Japanese Laid-open Patent Publication Nos. 62-211340 discloses a tungsten carbide/chromium carbide composite containing 31.about.84 wt % of tungsten carbide (however, the specifications only discloses examples containing 50-57 wt % of tungsten carbide), 15.about.60 wt % of TaC, NbC, ZrC, TiC, Cr.sub.3 C.sub.2, or MoC, and 1.about.9 wt % of Co or Ni. Japanese Laid-open Patent Publication Nos. 53-89807 discloses a tungsten carbide/chromium carbide composite containing 80+ wt % of tungsten carbide, less than 10 wt % of Cr.sub.3 C.sub.2, and 10 wt % of Co and no greater than 2 wt % of Cr and/or Ni.
Both composites disclosed in the '340 and '807 publications require metallic sintering agents, which cause the hardness of the sintered products to be reduced. Although the tungsten carbide/chromium carbide composite disclosed in the '340 publication contains mixtures of carbide and/or nitride compounds which allow the amount of metallic sintering aid required to be lowered, it still requires 1.about.9 percent of Co or Ni. Furthermore, homogeneity of the sintered composites has been a concern with the tungsten carbide/chromium carbide composite disclosed in the '340 publication. Another drawback experienced with the tungsten carbide based composites is that, in high-temperature applications, the metallic components are likely to soften, partially melt or even become partially oxidized. This could seriously and adversely affect the mechanical characteristic as well as the corrosion resistance of the final composite products, and thus undesirably limit the application of these composite materials.